"Google is currently failing emails sent to Gmail accounts if they are signed with a 512-bit key or less. Now there is evidence that Google is starting to enforce their policy of accepting DKIM keys of 1024-bit or higher."

"A mathematician recently cracked Google’s weak 512-bit DKIM key and impersonated founders Sergey Brin and Larry Page via email. A recent Wired.com article relating the story started a rush in the email industry to create new DKIM keys stronger than 512-bits. Google is taking this security issue seriously by requiring all senders to sign with a 1024-bit DKIM key."